Science case #4

Redshifted radio galaxies

Nearby starburst templates — Arp 220 at z = 0.018 and Arp 299 at z = 0.0109 — how long does its nuclear structure remain detectable when the same physical source is placed at cosmological distances and observed with SKA-MID?

Physical motivation

Local LIRGs and ULIRGs provide resolved laboratories for dusty, merger-driven star formation. Redshifting their radio structure tests when SKA-MID can still distinguish compact starburst morphology from an unresolved high-redshift source.

Physical evolution model

The redshift scaling follows Ghasemi-Nodehi et al. (2022): local radio-continuum maps are used as templates for how dusty star-forming galaxies would appear at higher redshift.

The apparent size is set by the angular-diameter distance:

\[ \theta(z) = \theta(z_0)\,\frac{D_A(z_0)}{D_A(z)} . \]

The integrated radio flux is scaled by distance, K-correction and optional SFR evolution:

\[ S_\nu(z) = S_\nu(z_0) \left[\frac{D_L(z_0)}{D_L(z)}\right]^2 \left[\frac{1+z}{1+z_0}\right]^{1-\alpha} f_{\rm SFR}(z) . \]

The SFR term follows the Schreiber et al. main sequence:

\[ f_{\rm SFR}(z) = \frac{{\rm SFR}_{\rm MS}(M_\star,z)} {{\rm SFR}_{\rm MS}(M_\star,z_{\rm ref})}, \] \[ \log_{10}{\rm SFR}_{\rm MS} = m - m_0 + a_0 r - a_1\left[\max(0,m-m_1-a_2 r)\right]^2, \] \[ m=\log_{10}\left(\frac{M_\star}{10^9 M_\odot}\right), \quad r=\log_{10}(1+z). \]

This page shows the simple total-emission case: one radio image, one global spectral scaling, and no intrinsic size evolution.

Input image models

The LIRG sky models were provided by Geferson Lucatelli iD (IAA-CSIC), from his high-resolution, multi-scale, multi-frequency observations of LIRGs; see his PhD thesis.

Radio model image of Arp 220

Arp 220

Late-stage ULIRG merger; distance ≈ 78 Mpc; IR-based SFR ≈ 200 M⊙ yr−1.

Radio model image of Arp 299

Arp 299

Interacting LIRG system; distance ≈ 45 Mpc; IR-based SFR ≈ 90 M⊙ yr−1.

Simulation runs

Telescope

Array
SKA-MID-AA*
Mode
Continuum snapshot

Observation

Snapshot length
10 minutes
Bands
1.35, 6.5 & 11.9 GHz

Redshift sequence

Source redshifts
Arp 299: 0.010 · Arp 220: 0.018
Targets
0.15, 0.3, 0.5, 1.0, 2.0

Products

Emission
Total radio continuum
Output
Simulated image sequences

Results

The source shrinks in angular size and loses flux rapidly, while the higher-frequency maps preserve the compact double-nucleus structure for longer at low redshift. At later epochs, the source becomes beam-limited and progressively blends into a compact detection.

Arp 220 redshift sequence at 1.35 GHz, 6.5 GHz and 11.9 GHz
Arp 220 redshift sequence from z = 0.15 to z = 2.00 across bands 2, 5a and 5b. Each row shares its intensity scale and has a colorbar on the right. The label in the top right of each panel is the redshift.
Arp 299 redshift sequence at 1.35 GHz, 6.5 GHz and 11.9 GHz
Arp 299 redshift sequence from z = 0.15 to z = 2.00 across bands 2, 5a and 5b. Each row shares its intensity scale and has a colorbar on the right. The label in the top right of each panel is the redshift.

Beyond the current model

This first implementation captures the main cosmological effects and provides a controlled baseline for comparing LIRG analogues across redshift. The same framework can be made progressively more realistic by adding further radio-continuum physics.

Takeaways